Pelletized silicone rubber

ABSTRACT

Pellets of a silicone rubber stock containing silicon-bonded vinyl radicals and a platinum catalyst and pellets of a silicone rubber stock containing a silicon compound containing at least three silicon-bonded hydrogen atoms per molecule are present in a mixture sufficient to provide a vulcanizable composition when massed. The vulcanizable composition is useful for extruding on wire and for other silicone rubber products.

United States Patent 1 1 Lamont 1 3,714,204 51 Jan. 30, 1973 1541PELLETIZED SILICONE RUBBER [75] Inventor: Peter Lamont, Midland, Mich.

[73] Assignee: Dow Corning Corporation, Midland,

Mich.

221 Filed: May22,l970 211 App1 No.:39,915

[52] US. Cl. ..260/37 SB, 260/46.5 G, 260/825 [51] Int. Cl ..C08g 51/04C08g 311/02 [58] Field of Search ..260/37 SB, 46.5 U, 46.5 P,

5/1959 France ..260/46;5

OTHER PUBLICATIONS Modern Plastics Encyclopedia & Engineers Hand book;1950; pp. 424427;'Sci. Lib., TP 986 A2M5.

Taylor-Stiles Plastic Pelleter; Index No, App. 210,

I 1955; pp. 1-4.

PrimaryExaminer Lewis T. Jacobs A!t0meyRobert F. Fleming, Jr Laurence R.Hobey,

Harry D. Dingman, Howard W. Hermann and Roger H. Borrousch [57] ABSTRACTPellets of a silicone rubber stock containing siliconbonded vinylradicals and a platinum catalyst and pellets of a silicone rubber stockcontaining a silicon compound containing at least three silicon-bondedhydrogen atoms per molecule are present in a mixture sufficient toprovide a vulcanizable composition when massed. The vulcanizablecomposition is useful for extruding on wire and for other siliconerubber products.

43 Claims, No Drawings PELLETIZED SILICONE RUBBER This invention relatesto a, silicone rubber composi= tion in pelletized form.

The cure system involving the addition of siliconbonded hydrogen atomsto aliphatically unsaturated bonds in the presence of a catalyst hasfound broad utility in silicone preparations and curing processes. Suchprocesses are described by Speier et al. in U.S. Pat. No. 2,823,218.This system has been used in the preparation of silicone rubbers wheresilicon-bonded hydrogen atoms add to silicon-bonded vinyl radicals inthe presence of a platinum catalyst. The present invention relates tothese silicone rubbers. Silicone rubbers Y of this type have foundcommercial acceptance in low temperature curing two part roomtemperature vulcanizing applications. Such compositions are low inviscosity and are suitable in encapsulating electrical components and aspotting compounds and sealants. More recently, these compositions havebeen made into one part curing systems where the platinum catalyst isinhibited at low temperatures but is active upon heating. Such aninhibited systemis described by Moore in U.S. Pat. No. 3,192,181 inwhich benzotriazole inhibits the cure at temperatures up to about 60 C.but cures readily when heated above 60 C.

The physical properties of silicone rubbers cured by the silicon-bondedhydrogen atom addition to siliconbonded vinyl radicals in the presenceof a platinum catalyst can be varied broadly by changing the type andamount of the reactants, such as the silicon compound containing thesilicon-bonded hydrogen atoms. Thus, a silicone rubber can be readilytailored for specific uses. These silicone rubbers cure at roomtemperature, but can be inhibited from doing so. The rate ofvulcanization is sensitive to increases in temperature and more so thanperoxide catalyst. For these reasons, it is desirable to use this curesystem in silicone rubbers which are based on gums.

The use of a cure system, for silicone rubbers based on gums, which curethrough the addition of siliconbonded hydrogen atoms to silicon-bondedvinyl radicals in the presence of a platinum catalyst, as such, is notreadily adaptable to wide applications, because the shelf life of such acatalyzed silicone rubber stock is short since curing begins as soon asthe catalyst is added and because adding the platinum'catalyst justbefore use requires milling and heating if thorough mixing is to beachieved and heating only accelerates the curing process. These factorshave prevented any broad utility of these types of silicone rubbers. Themore recent discovery of inhibitors for the platinum catalyst hasbroadened the scope of utility somewhat. However, inhibitors onlyinhibit the cure up to 60f C. or 70 C., some inhibitors are volatile andthus, the same convenience 'as found with peroxide catalyst has not beenachieved even with inhibitors, since peroxides are stable or inactive upto temperatures in excess of 100 C. with some as high as 130 C. or 150C. Substances which inhibit the activity of the platinum catalysts attemperatures above 60 C. or 70 C. do so permanently and are classed aspoisons.

Therefore, it is seen that a platinum cure system for silicone rubbergum stocks is desirable, however, the handling problems limit theircommercial use. Platinum cure systems also have the advantage of notbeing inhibited by air which is true with some peroxides. lt istherefore an object of the present invention to provide a siliconerubber stock which can be used as peroxide cured stocks but cured with aplatinum catalyst. Thisand other objects will become apparent from thefollowing detailed description of the present invention.

This invention relates to a curable composition in pelletized formconsisting essentially of (A) pellets consisting essentially of asilicone rubber stock containing silicon-bonded vinyl radicals and aplatinum catalyst and (B) pellets consisting essentially of a siliconerubber stock containing a silicon compound containing at least threesilicon-bonded hydrogen atoms per molecule, pellets (A) and pellets (B)present in amounts sufficient to provide a vulcanizable composition whenmassed.

A silicone rubber stock, for the purposes of the present invention, isany silicone gum with or without fillers and other additives. Thesilicone rubber stocks of the present invention include everythingexcept a curing catalyst. The silicone rubber stocks in the presentinvention are also limited to those stocks which contain silicone gumswith silicon-bonded vinyl radicals. Such vinyl containing siliconerubber stocks are well known in the art and are available commercially.The vinyl containing silicone, rubber stocks suitable for the presentinvention are those which can be pelletized by any of a number ofmethods. The suitable silicone rubber stocks which can be pelletizedwill be apparent to any person skilled in the art of silicone rubber.Those silicone rubber stocks which flow at ambient conditions,naturally, are unsuitable, since such silicone rubber stocks would notremain pellets long and would coagulate and lump together upon storageor shipping and thus any advantages gained by pell'eting would be lost.The silicone rubber stocks which do not flow but which are slightlytacky, are suitable for the present invention if they are slightlypowered with an inert powder, such as talc. Finely divided or powderedextending fillers can be used to powder these slightly tacky siliconerubber stocks after pelletization. This permits their use and keeps themfree flowing. For the purposes of the present invention, pellets arediscrete particles which can vary broadly in size from about the size ofgrains of sand to two or three inch diameter particles. The particlesneed not be in any regular form, such as spheres.

The curable composition of the present invention which is in pelletizedform consists essentially of a mixture of two types of pellets. One typeof pellet, (A), consists essentially of a silicone rubber stock whichcontains silicon-bonded vinyl radicals and a platinum catalyst. Theother type of pellet, (B), contains a silicone rubber stock whichcontains silicon-bonded vinyl radicals and a silicon compound whichcontains at least three silicon-bonded hydrogen atoms per molecule. Thesilicone rubber stock of (A) can be the same as the silicone rubberstock of (B) or it can be different, preferably the silicone rubberstocks of (A) and (B) are the same. When the silicone rubber stocks arethe same, the final total composition, (A) plus (B), is readilydetermined and the amount of platinum catalyst and silicon-bondedhydrogen atoms in the total composition can be readily altered by merelychanging the ratio of (A) and (B) in the mixture of pellets.

For the most part, the silicone rubber stocks containing thesilicon-bonded vinyl radicals consists essentially of adiorganopolysiloxane gum containing siliconbonded vinyl radicals. Thediorganopolysiloxane gums are units of the formula R,Si bonded togetherby silicon oxygen-silicon bonds in which R is a monovalent hydrocarbonradical or a halogenated monovalent hydrocarbon radical. Preferably,there are at least two vinyl radicals per molecule. The maximum numberof vinyl radicals per molecule is not critical, since as the number ofvinyl radicals increase the cured product becomes more highlycross-linked or the amount of silicon-bonded hydrogen atoms in pellets(B) can be reduced to provide the cross-linking desired.

R can be any monovalent hydrocarbon radical such as alkyl radicals suchas methyl, ethyl, propyl, isopropyl, butyl, pentyl, isopentyl,neopentyl, hexyl, octyl, dodecyl, octadecyl, 3-methylheptyl,6-butyloctadecyl tertiary butyl, myricyl and 2,2-diethylpentyl; alkenylradicals such as, vinyl, allyl, hexenyl, butenyl, 3- octenyl,4,9octadecadienyl and 4-nonenyl; alkynyl radicals such as propynyl,heptynyl, butynyl, decynyl; alkenynyl radicals such as l-penten-3-ynyl,2-ethyl-1- buten-3-ynyl; cycloaliphatic radicals such as, cyclobutyl,cyclopentyl, cyclohexyl, cycloheptyl, propylcyclohexyl,2,4-dimethylcyclopentyl, cyclohexenyl, bicyclo[3.l.0]hexyl,tricyclo[3.2.l.l "]--nonenyl, Spiro-[4.5 ]decyl, dispiro[4. l .4.2]tridecenyl, decahydronaphthyl, 2,3-dihydroindyl and l,2,3,4-tetrahydronaphthyl; aryl radicals such as phenyl, tolyl, xylyl,3-ethylphenyl, xenyl, naphthyl, anthracyl, pentacenyl,3,4-methylethylphenyl, 9,9'-bifluoryl and 4-mterphenyl; and aralkylradicals such as Z-phenyI-octyl, 3-methyl-2-(4-isopropylphenyl)heptyl,benzyl, 2-ethyltolyl, Z-ethylp-cymyl, diphenylmethyl,4,5-diphenylpentyl, Z-phenylethyl and 2-phenylpropyl.

R can be any monovalent halogenated hydrocarbon group such as aliphaticgroups such as chloromethyl, 3- chloropropyl, 3,3,3-trichloropropyl,perfluorovinyl, chlorooctadecyl or radicals of the formula R,CH CH,-where R, can be any perfluoroalkyl group such as trifluoromethyl,perfluoroethyl, perfluoroisobutyl, perfluoro-heptyl orperfluorooctadecyl; aromatic groups such as dichlorophenyl,tetrabromoxenyl, tetrachlorophenyl, alpha,alpha,alpha,-trifluorotolyl oriodonaphthyl; cycloaliphatic groups such as chlorocyclohexyl,bromocyclopentyl or chlorocyclohexenyl and aralkyl groups such aschlorobenzyl, beta (chlorophenyl)ethyl or beta-(iodophenyl)ethyl orbeta- (bromophenyl)propyl.

Preferably, R is methyl and vinyl radicals. The diorganopolysiloxanegums can be endblocked with siliconbonded hydroxyl radicals ortriorganosiloxy radicals where the organic groups are the same as for R.

The silicone rubber stocks can also contain fillers. The fillers arethose conventionally used in silicone rubber. The fillers can be eitherreinforcing fillers such as fume silica or non-reinforcing fillers. Thesilica fillers can be treated or untreated to make them hydrophobic.Treated fillers are well known in the art and include treatment withchlorosilanes, such as triorganochlorosilanes anddiorganodichlorosilane, diorganocyclosilo'xanes, hexaorganodisiloxane,organosilazanes and the like. Examples of suitable fillers include, fumesilica, silica aerogel, silica xerogel, silica soot, carbon black,quartz, diatomaceous earth, metal carbonates such as calcium carbonate,metal oxides, such as alumina, metal silicates such as zirconiumsilicate, clays, talc and the like. The silicone rubber stockspreferably contain fillers, particularly, silica fillers to reinforcethe rubber and provide increased tensile and tear properties. The use offillers also provide a 5 wider range of diorganopolysiloxane gums whichcan be used, such as those of lower plasticity.

The silicone rubber stocks can also contain other ingredientsconventionally used in silicone rubber, such.

as pigments to impart color, compression set additives, plasticizers,heat stability additives and the like.

The platinum catalysts are well known and any of the platinum catalystswhich are readily dispersible in the silicone rubber stock are suitablefor the present inven- PtClaCO and PtBrzCO The platinum catalysts canalso have presentinhibitors such as the benzotriazole mentioned above,astannous salt, a mercuric salt, a bismuth salt, a cuprous salt, acupric salt, an acetylenic unsaturated compound such as2-ethynylisopropanol and the like. The inhibitors, however, are notessential to the present invention.

The silicon compound containing at least three silicon-bonded hydrogenatoms can be any of those well known silicon compounds which contain atleast three siliconbonded hydrogen atoms. These silicon compounds, forthe most part, contain silicon-bonded organic groups as defined for Rabove and are free of aliphatic unsaturation. The silicon compounds canbe illustrated by compounds composed of one or more of the units HSiORHSiO, R,HSiO and SiO where R is defined above, preferably R is methyl,phenyl or 3,3,3-trifluoropropyl. Illustrative of the silicon compoundscontaining at least three silicon-bonded hydrogen atoms are HSi [OSi(CH),H],OSi(CH,) Q h a)a a) ]io a)a, [(CH )HSiO] Si[OSi(CH ),H],,copolymers of SiO, and H(CH ),SiO copolymers of SiO (CH ),SiO and H(CH),SiO copolymers of cng sio H(CH,,),S i0 and (CH ),SiO, copolymers of(CH )HSiO and (C,H,)(CH;,)SiO, copolymers of (CH )HSiO, (CH,),Si O and(C,H,,),Si0, copolymers of CHgHSiO and ah s l 5)( 1a usi osi Ohiill ,cnol im-isurminimum I ("sun "CH3 :1 i

platinum complexes and chloroplatinic acid. The

(CH1 S10 and (CHQHSiU and oF IncInsuosucnnnu.

The diorganopolysiloxane gums should have a plasticity of at least 0.050inch. Diorganopolysiloxanes which have plasticities between 0.050 inchto about 0.070 inch require filler present to provide a silicone rubberstock which does not flow and which can be pelletized. The siliconerubber stocks preferably contain a filler when the diorganopolysiloxanegum has a plasticity between 0.050 and 0.100 inch. As a rule of thumb,the higher the plasticity the less filler is required to provide apelletizable silicone rubber stock. However, filler need not beexcluded, regardless of the plasticity, since filler, particularly thereinforcing fillers, are advantageously used to enhance the physicalproperties. The silicone rubber stocks which are readily pelletized havea plasticity of at least 0.100 inch. The plasticity is determined byASTM-D-926-56 test procedure.

The pellets (A) and pellets (B) are mixed in the proper proportions togive a vulcanizable product. To give reasonable rubber properties thereshould be at least 1.0 silicon-bonded hydrogen atom per v'inyl radical.The upper limit is not narrowly critical but should not exceed onesilicon-bonded hydrogen-bearing silicon compound molecule per vinylradical. If one exceeds one silicon-bonded hydrogen-bearing siliconcompound molecule per vinyl radical, the silicone rubber stock will notvulcanize.

Silicone rubber stocks which are most suitable for purposes of thepresent invention are those which contain from to 100 inclusive parts byweight of a reinforcing silica filler per 100 parts by weight of thediorganopolysiloxane gum and the platinum catalyst is present in anamount sufficient to provide from 10 to 150 parts by weight platinum perone million parts by weight of diorganopolysiloxane gum. Whennon-reinforcing fillers are used the total amount of filler, bothreinforcing and non-reinforcing, can be up to 300 parts by weight per100 parts by weight diorganopolysiloxane gum. Preferably, the amount ofreinforcing silica filler is from 30 to parts by weight per parts byweight of diorganopolysiloxane gum.

The silicone rubber stock, both for (A) and (B), can be pelletized by anumber of methods. The silicone rubber stocks can be pelletized bycommercial pel letizers or granulators. They also can be pelletized,especially for the small size pellets, by spray drying, such as bydispersing the silicone rubber stock in an organic solvent and sprayinginto a heated atmosphere such that the solvent is rapidly vaporizedleaving discrete pellets behind. Commercial spray dryers are suitablefor this technique. The silicone rubber stocks can also be pelletized byextruding the stock through an orifice and chopping the extruded stockto suitable sizes. One can also sheet and chop to produce the pellets.lf the particular silicone rubber stock is slightly tacky, the pelletsshould be powdered immediately with talc or some substitute to avoid anycoagulating or lumping.

The pelletized compositions of the present invention are particularlyuseful in extrusion applications because of their easy handling.Thepresent pelletized composition is a two component system, having allthe stability of two component systems, but usable as a one componentsystem, since the two components can be stored and shipped in onepackage. The pelletized composition can be extruded without firstmilling it. The one package, two component composition is stable untilit is massed, by extruding, by milling or other massing means. Thepelletized composition eliminates steps in processes using siliconerubber, such as the fabricating of silicone rubber parts, the extrusionof silicone rubber on wire and the like. The pelletized compositioncould be poured directly from a storage container into a hopper feedextruder without the time consuming milling operation. The pelletizedcomposition can be extruded on fabric, can be molded and used in anyother way silicone rubber gum stocks can be used with improved handling,however.

The following examples are for illustrative purposes only and should notbe construed as limiting the present invention which is properlydelineated in the claims.

EXAMPLE 1 A mixture of 100 parts by weight of a diorganopolysiloxane gumhaving 99.858 mol per cent dimethylsiloxane units and 0.142 mol per centmethylvinylsiloxane units, endblocked with dimethylvinylsiloxane unitsand having a plasticity of 0.060 inch, 55 parts by weight of areinforcing silica filler, 7 parts by weight of a hydroxyl endblockedpolydimethylsiloxane fluid having a viscosity of 2 cs. at 25 C., 5 partsby weight of diphenylsilanediol and 1 part by weight of a hydroxylend-blocked polymethylvinylsiloxane fluid having 4 weight per centhydroxyl radicals, was mixed and heated for 3 hours at to C. undervacuum. This mixture was divided and two mixtures were prepared bymilling. Mixture (A) was 100 parts by'weight of the above gum mixture,79 X 10' parts by weight platinum added as a 0.19 weight per centchloroplatinic acid solution in butyl carbitol acetate and 0.36 part byweight'of methylbutynol. Mixture (B) was 100 parts by weight of theabove gum mixture, 1.8 parts of a heat stability additive and 3.0 partsby weight of a benzene soluble copolymer having dimethylhydrogensiloxaneunits and SiO units in a mole ratio of 2 to 3.36 respectively. Mixture(A) and mixture (B) were separately extruded through an orifice havingfour one-quarter inch diameter openings and as the extruded mixtureimmerges from the openings a blade cut the extrusions in approximatelyonequarter inch lengths. The pellets were allowed to fall into a talepowder. The pellets were then separated from the excess talc powder byscreening. The pellets of mixture (A) were mixed with pellets of mixture(B) in equal weight. A part of the mixed pellet composition was massedby milling and then press molded for 10 minutes at 171 C. The moldedarticle had a durometer on theShore A scale of 72, a tensile strength atbreak of 1,225 p.s.i. and an elongation of 380 per cent. Another part ofthe mixed pellet composition was extruded on wire and hot air vulcanizedto give silicone rubber with a tensile strength at break of 1,171 p.s.i.and an elongation at break of 380 per cent.

EXAMPLE 2 A mixture, (A), was prepared by milling together 100 parts byweight of the diorganopolysiloxane gurn described in Example 1, 55 partsby weight of a reinforcing silica filler, 8.5 parts by weight of 5micron quartz, 7 parts by weight of a hydroxyl endblockedpolydimethylsiloxane fluid having a viscosity of 2 cs. at 25C., 5 partsby weight of diphenylsilanediol, 1 part by weight of a hydroxylendblocked polymethylvinylsiloxane fluid having 4 weight percenthydroxyl radicals, 1.5 parts by weight of a heat stability additive and1 part by weight of a chloroplatinic acid catalyst having 0.22 weightper cent platinum.

A mixture, (B), was prepared by milling silicone rubber stock asdescribed above except in place of the platinum catalyst, 1.5 parts byweight of a trimethylsiloxane endblocked polymethylhydrogensiloxanefluid having an average of about 30 siloxane units per molecule wasadded.

Mixtures (A) and (B) were separately put into a chlorothene dispersionto obtain about 1 1.7 weight per cent solids. The chlorothene dispersionwas then sprayed through a nozzle into a heated air atmosphere at about150 to 175C. and a pressure of about 100 p.s.i. The spray dried pelletsof silicone rubber stock were about 80 to 100 microns across. Pellets ofmixture (A) were mixed with pellets of mixture (B) in equal weights andstored for 7 months. The stored pellets I remained separated and werereadily massed by milling for 1 minute. The massed pellets were thenmilled for 4 minutes more. The milledmixture was press molded for 10minutes at 149 C. The vulcanized silicone rubber had a durometer of 42on the Shore A scale, a tensile strength at break of 80 p.s.i., anelongation at break of 580 per cent and a tear strength, die B of 28p.s.i.

That which is claimed is: l. A curable composition in pelletized formconsisting essentially of the admixture of (A) pellets consistingessentially of a silicone rubber stock containing silicon-bonded vinylradicals and a platinum catalyst and (B) pellets consisting essentiallyof a silicone rubber stock containing a silicon compound containing atleast three silicon-bonded hydrogen atoms per molecule, pellets (A) andpellets (B) present in amounts sufficient to provide a vulcanizablecomposition when massed.

2. The curable composition in pelletized form in accordance with claim 1in which the silicone rubber stock of pellets (A) consists essentiallyof a diorganopolysiloxane gum having a unit formula R SiO in which R isa monovalent radical selected from the group consisting of a hydrocarbonradical and a halogenated hydrocarbon radical, said units being bondedtogether by silicon-oxygen-silicon bonds, there being at least two vinylradicals per molecule and said diorganopolysiloxane gum being endblockedwith radicals selected from the group consisting of a hydroxyl radicaland triorganosiloxy radicals where the organic radicals are R radicalsand a filler,'and the silicone rubber stock of pellets (B) consistsessentially ofa diorganopolysiloxane gum as described for pellets (A)and a filler.

,3. The curable composition in pelletized form in accordance with claim2 in which R radicals other than vinyl are methyl.

4. The curable composition in pelletized form in accordance with claim 2in which R radicals other than vinyl are methyl radicals and phenylradicals and not more than percent of the R radicals being phenylradicals.

5. The curable composition in pelletized form in accordance with claim 2in which the plasticity of the diorganopolysiloxane gum is at least0.050 inch.

6. The curable composition in pelletized form in accordance with claim 3in which the plasticity of the diorganopolysiloxane gum is at least0.050 inch.

7. The curable composition in pelletized form in accordance with claim 4in which the plasticity of the diorganopolysiloxane gum is at least0.050 inch.

8. The curable composition in pelletized form in accordance with claim 1in which the silicon compound containing at least three silicon-bondedhydrogenatoms per molecule having in addition an average of up to twomonovalent organic radicals per silicon atom selected from the groupconsisting of monovalent hydrocarbon radicals free of aliphaticunsaturation and monovalent halogenated hydrocarbon radicals free ofaliphatic unsaturation, the remaining valences of the silicon atomsbeing satisfied by divalent oxygen atoms linking the silicon atomstogether.

9. The curable composition in pelletized form in accordance with claim 2in which the silicon compound containing at least three silicon-bondedhydrogen atoms per molecule having in addition an average of up totwomonovalent organic radicals per silicon atom selected from the groupconsisting of monovalent hydrocarbon radicals free of aliphaticunsaturation and monovalent halogenated hydrocarbon radicals free ofaliphatic unsaturation, the remaining valences of the silicon atomsbeing satisfied by divalent oxygen atoms linking the silicon atomstogether.

10. The curable composition in pelletized form in accordance with claim9 in which the R radicals and the organic radicals other than vinyl aremethyl.

11. The curable composition in pelletized form in accordance with claim9 in which the R radicals and the organic radicals other than vinyl aremethyl radicals and phenyl radicals and not more than 50 percent of theR radicals being phenyl radicals.

12. The curable composition in pelletized form in accordance with claim9 in which the plasticity of the diorganopolysiloxane gum is at least0.050 inch.

13. The curable composition in pelletized form in ac cordance with claim10 in which the plasticity of the diorganopolysiloxane gum is at least0.050 inch.

14. The curable composition in pelletized form in accordance with claim11 in which the plasticity of the diorganopolysiloxane gum is at least0.050 inch.

15. The curable composition in accordance with claim 1 in which theratio of silicon-bonded hydrogen atoms per silicon-bonded vinyl radicalsis at least 1.0 and up to one silicon-bonded hydrogen bearing siliconcompound molecule per vinyl radical.

16. The curable composition in pelletized form in accordance with claim5 in which the ratio of siliconbonded hydrogen atoms per silicon-bondedvinyl radicals is at least 1.0 and up to one silicon-bonded hydrogenbearing silicon compound molecule per vinyl radical.

17. The curable composition in pelletized form in accordance with claim13 in which the ratio of siliconbonded hydrogen atoms per silicon-bondedvinyl radicals is at least 1.0 and up to one silicon-bonded hydrogenbearing silicon compound molecule per vinyl radical.

18. The curable composition in pelletized form in accordance with claim1 in which each pellet has a plasticity of at least 0.100 inch.

19. The curable composition in pelletized form in accordance with claim2 in which each pellet has a plasticity of at least 0.100 inch.

20. The curable composition in pelletized form in accordance with claim12 in which each pellet has a plasticity of at least 0.100 inch.

21. The curable composition in pelletized form in accordance with claim17 in which each pellet has a plasticity of at least 0.100 inch.

22. The curable composition in pelletized form in accordance with claim2 in which the filler in the total composition is a reinforcing silicafiller present in an amount from to 100 inclusive parts by weight per100 parts by weight of the diorganopolysiloxane gum and the platinumcatalyst in the total composition is present in an amount to providefrom 10 to 150 inclusive parts by weight platinum per one million partsby weight of the diorganopolysiloxane gum.

23. The curable composition in pelletized form in accordance with claim6 in which the filler in the total composition is a reinforcing silicafiller present in an amount from 10 to 100 inclusive parts by weight per100 parts by weight of the diorganopolysiloxane gum and the platinumcatalyst in the total composition is present in an amount to providefrom 10 to 150 inclusive parts by weight platinum per one million partsby weight of the diorganopolysiloxane gum.

24. The curable composition in pelletized form in accordance with claim7 in which the filler in the total composition is a reinforcing silicafiller present in an amount from 10 to 100 inclusive parts by weight per100 parts by weight of the diorganopolysiloxane gum and the platinumcatalyst in the total composition is present in an amount to providefrom 10 to 150 inclusive parts by weight platinum per one million partsby weight of the diorganopolysiloxane gum.

25. The curable composition in pelletized form in accordance with claim16 in which the filler in the total composition is a reinforcing silicafiller present in an amount from 10 to 100 inclusive parts by weight per100 parts by weight of the diorganopolysiloxane gum and the platinumcatalyst in the total composition is present in an amount to providefrom 10 to 150 inclusive parts by weight platinum per one million partsby weight of the diorganopolysiloxane gum.

26. The curable composition in pelletized form in accordance with claim21 in which the filler in the total composition is a reinforcing silicafiller present in an amount from 10 to 100 inclusive parts by weight per100 parts by weight of the diorganopolysiloxane gum and the platinumcatalyst in the total composition is present in an amount to providefrom 10 to 150 inclusive parts by weight platinum per one million partsby weight of the diorganopolysiloxane gum.

27. The curable composition in pelletized form in accordance with claim22 in which an extending filler is also present to provide up to 300parts by weight total filler per parts by weight of diorganopolysiloxanegum in the total composition.

28. The curable composition in pelletized form in accordance with claim23 in which an extending filler is also present to provide up to 300parts by weight total filler per 100 parts by weight ofdiorganopolysiloxane gum in the total composition.

29. The curable composition in pelletized form in accordance'with claim24 in which an extending filler is also present to provide up to 300parts by weight total filler per 100 parts by weight ofdiorganopolysiloxane gum in the total composition.

30. The curable composition in pelletized form in accordance with claim25 in which an extending filler is also present to provide up to"300parts by weight total filler per 100 parts by weight ofdiorganopolysiloxane gum in the total composition.

31. The curable composition in pelletized form in accordance with claim26 in which an extending filler is also present to provide up to 300parts by weight total filler per 100 parts by weight of diorganosiloxanegum in the total composition.

32. The curable composition in pelletized form in accordance with claim26 in which the reinforcing silica filler is present in an amount offrom 30 to 60 inclusive parts by weight per 100 parts by weight of thediorganopolysiloxane gum. I

33. The curable composition in pelletized form in accordance with claim1 in which the silicone rubber stock of pellets (A) and pellets (B) arethe same.

34. The curable composition in pelletized form in accordance with claim2 in which the silicone rubber stock of pellets (A) and pellets (B) arethe same.

35. The curable composition in pelletized form in accordance with claim6 in which the silicone rubber stock of pellets (A) and pellets (B) arethe same.

36. The curable composition in pelletized form in accordance with claim21 in which the silicone rubber stock of pellets (A) and pellets (B) arethe same.

37. The curable composition in pelletized form in accordance with claim26 in which the silicone rubber stock of pellets (A) and pellets (B) arethe same.

38. The curable composition in pelletized form in accordance with claim31 in which the silicone rubber stock of pellets (A) and pellets (B) arethe same.

39. The curable composition in pelletized form in accordance with claim2 in which a plasticizer is also present.

40. The curable composition in pelletized form in accordance with claim21 in which a plasticizer is also present.

41 The curable composition in pelletized form in accordance with claim37 in which a plasticizer is also present.

42. A method comprising (1) pelletizing a silicone rubber stockcontaining silicon-bonded vinyl radicals and a platinum catalyst, (ll)pelletizing a silicone rubber stockcontaining a silicon compoundcontaining at least three silicon-bonded hydrogen atoms per molecule,(III) mixing the pellets produced in (I) and the pellets produced in(ll) in amounts sufficient to provide a vulcanizable composition whenmassed.

43. A curable composition in pelletized form consisting essentially ofthe admixture of hydrogen atoms per molecule and in which the organicradicals are selected from monovalent hydrocarbon radicals andmonovalent halogenated hydrocarbon radicals, pellets (A) and pellets (8)present in amounts sufficient to provide a vulcanizable composition.

1. A curable composition in pelletized form consisting essentially ofthe admixture of (A) pellets consisting essentially of a silicone rubberstock containing silicon-bonded vinyl radicals and a platinum catalystand (B) pellets consisting essentially of a silicone rubber stockcontaining a silicon compound containing at least three silicon-bondedhydrogen atoms per molecule, pellets (A) and pellets (B) present inamounts sufficient to provide a vulcanizable composition when massed. 2.The curable composition in pelletized form in accordance with claim 1 inwhich the silicone rubber stock of pellets (A) consists essentially of adiorganopolysiloxane gum having a unit formula R2SiO in which R is amonovalent radical selected from the group consisting of a hydrocarbonradical and a halogenated hydrocarbon radical, said units being bondedtogether by silicon-oxygen-silicon bonds, there being at least two vinylradicals per molecule and said diorganopolysiloxane gum being endblockedwith radicals selected from the group consisting of a hydroxyl radicaland triorganosiloxy radicals where the organic radicals are R radicalsand a filler, and the silicone rubber stock of pellets (B) consistsessentially of a diorganopolysiloxane gum as described for pellets (A)and a filler.
 3. The curable composition in pelletized form inaccordance with claim 2 in which R radicals other than vinyl are methyl.4. The curable composition in pelletized form in accordance with claim 2in which R radicals other than vinyl are methyl radicals and phenylradicals and not more than 50 percent of the R radicals being phenylradicals.
 5. The curable composition in pelletized form in accordancewith claim 2 in which the plasticity of the diorganopolysiloxane gum isat least 0.050 inch.
 6. The curable composition in pelletized form inaccordance with claim 3 in which the plasticity of thediorganopolysiloxane gum is at least 0.050 inch.
 7. The curablecomposition in pelletized form in accordance with claim 4 in which theplasticity of the diorganopolysiloxane gum is at least 0.050 inch. 8.The curable composition in pelletized form in accordance with claim 1 inwhich the silicon compound containing at least three silicon-bondedhydrogen atoms per molecule having in addition an average of up to twomonovalent organic radicals per silicon atom selected from the groupconsisting of monovalent hydrocarbon radicals free of aliphaticunsaturation and monovalent halogenated hydrocarbon radicals free ofaliphatic unsaturation, the remaining valences of the silicon atomsbeing satisfied by divalent oxygen atoms linking the silicon atomstogether.
 9. The curable composition in pelletized form in accordancewith claim 2 in which the silicon compound containing at least threesilicon-bonded hydrogen atoms per molecule having in addition an averageof up to two monovalent organic radicals per silicon atom selected fromthe group consisting of monovalent hydrocarbon radicals free ofaliphatic unsaturation and monovalent halogenated hydrocarbon radicalsfree of aliphatic unsaturation, the remaining valences of the siliconatoms being satisfied by divalent oxygen atoms linking the silicon atomstogether.
 10. The curable composition in pelletized form in accordancewith claim 9 in which the R radicals and the organic Radicals other thanvinyl are methyl.
 11. The curable composition in pelletized form inaccordance with claim 9 in which the R radicals and the organic radicalsother than vinyl are methyl radicals and phenyl radicals and not morethan 50 percent of the R radicals being phenyl radicals.
 12. The curablecomposition in pelletized form in accordance with claim 9 in which theplasticity of the diorganopolysiloxane gum is at least 0.050 inch. 13.The curable composition in pelletized form in accordance with claim 10in which the plasticity of the diorganopolysiloxane gum is at least0.050 inch.
 14. The curable composition in pelletized form in accordancewith claim 11 in which the plasticity of the diorganopolysiloxane gum isat least 0.050 inch.
 15. The curable composition in accordance withclaim 1 in which the ratio of silicon-bonded hydrogen atoms persilicon-bonded vinyl radicals is at least 1.0 and up to onesilicon-bonded hydrogen bearing silicon compound molecule per vinylradical.
 16. The curable composition in pelletized form in accordancewith claim 5 in which the ratio of silicon-bonded hydrogen atoms persilicon-bonded vinyl radicals is at least 1.0 and up to onesilicon-bonded hydrogen bearing silicon compound molecule per vinylradical.
 17. The curable composition in pelletized form in accordancewith claim 13 in which the ratio of silicon-bonded hydrogen atoms persilicon-bonded vinyl radicals is at least 1.0 and up to onesilicon-bonded hydrogen bearing silicon compound molecule per vinylradical.
 18. The curable composition in pelletized form in accordancewith claim 1 in which each pellet has a plasticity of at least 0.100inch.
 19. The curable composition in pelletized form in accordance withclaim 2 in which each pellet has a plasticity of at least 0.100 inch.20. The curable composition in pelletized form in accordance with claim12 in which each pellet has a plasticity of at least 0.100 inch.
 21. Thecurable composition in pelletized form in accordance with claim 17 inwhich each pellet has a plasticity of at least 0.100 inch.
 22. Thecurable composition in pelletized form in accordance with claim 2 inwhich the filler in the total composition is a reinforcing silica fillerpresent in an amount from 10 to 100 inclusive parts by weight per 100parts by weight of the diorganopolysiloxane gum and the platinumcatalyst in the total composition is present in an amount to providefrom 10 to 150 inclusive parts by weight platinum per one million partsby weight of the diorganopolysiloxane gum.
 23. The curable compositionin pelletized form in accordance with claim 6 in which the filler in thetotal composition is a reinforcing silica filler present in an amountfrom 10 to 100 inclusive parts by weight per 100 parts by weight of thediorganopolysiloxane gum and the platinum catalyst in the totalcomposition is present in an amount to provide from 10 to 150 inclusiveparts by weight platinum per one million parts by weight of thediorganopolysiloxane gum.
 24. The curable composition in pelletized formin accordance with claim 7 in which the filler in the total compositionis a reinforcing silica filler present in an amount from 10 to 100inclusive parts by weight per 100 parts by weight of thediorganopolysiloxane gum and the platinum catalyst in the totalcomposition is present in an amount to provide from 10 to 150 inclusiveparts by weight platinum per one million parts by weight of thediorganopolysiloxane gum.
 25. The curable composition in pelletized formin accordance with claim 16 in which the filler in the total compositionis a reinforcing silica filler present in an amount from 10 to 100inclusive parts by weight per 100 parts by weight of thediorganopolysiloxane gum and the platinum catalyst in the totalcomposition is present in an aMount to provide from 10 to 150 inclusiveparts by weight platinum per one million parts by weight of thediorganopolysiloxane gum.
 26. The curable composition in pelletized formin accordance with claim 21 in which the filler in the total compositionis a reinforcing silica filler present in an amount from 10 to 100inclusive parts by weight per 100 parts by weight of thediorganopolysiloxane gum and the platinum catalyst in the totalcomposition is present in an amount to provide from 10 to 150 inclusiveparts by weight platinum per one million parts by weight of thediorganopolysiloxane gum.
 27. The curable composition in pelletized formin accordance with claim 22 in which an extending filler is also presentto provide up to 300 parts by weight total filler per 100 parts byweight of diorganopolysiloxane gum in the total composition.
 28. Thecurable composition in pelletized form in accordance with claim 23 inwhich an extending filler is also present to provide up to 300 parts byweight total filler per 100 parts by weight of diorganopolysiloxane gumin the total composition.
 29. The curable composition in pelletized formin accordance with claim 24 in which an extending filler is also presentto provide up to 300 parts by weight total filler per 100 parts byweight of diorganopolysiloxane gum in the total composition.
 30. Thecurable composition in pelletized form in accordance with claim 25 inwhich an extending filler is also present to provide up to 300 parts byweight total filler per 100 parts by weight of diorganopolysiloxane gumin the total composition.
 31. The curable composition in pelletized formin accordance with claim 26 in which an extending filler is also presentto provide up to 300 parts by weight total filler per 100 parts byweight of diorganosiloxane gum in the total composition.
 32. The curablecomposition in pelletized form in accordance with claim 26 in which thereinforcing silica filler is present in an amount of from 30 to 60inclusive parts by weight per 100 parts by weight of thediorganopolysiloxane gum.
 33. The curable composition in pelletized formin accordance with claim 1 in which the silicone rubber stock of pellets(A) and pellets (B) are the same.
 34. The curable composition inpelletized form in accordance with claim 2 in which the silicone rubberstock of pellets (A) and pellets (B) are the same.
 35. The curablecomposition in pelletized form in accordance with claim 6 in which thesilicone rubber stock of pellets (A) and pellets (B) are the same. 36.The curable composition in pelletized form in accordance with claim 21in which the silicone rubber stock of pellets (A) and pellets (B) arethe same.
 37. The curable composition in pelletized form in accordancewith claim 26 in which the silicone rubber stock of pellets (A) andpellets (B) are the same.
 38. The curable composition in pelletized formin accordance with claim 31 in which the silicone rubber stock ofpellets (A) and pellets (B) are the same.
 39. The curable composition inpelletized form in accordance with claim 2 in which a plasticizer isalso present.
 40. The curable composition in pelletized form inaccordance with claim 21 in which a plasticizer is also present.
 41. Thecurable composition in pelletized form in accordance with claim 37 inwhich a plasticizer is also present.
 42. A method comprising (I)pelletizing a silicone rubber stock containing silicon-bonded vinylradicals and a platinum catalyst, (II) pelletizing a silicone rubberstock containing a silicon compound containing at least threesilicon-bonded hydrogen atoms per molecule, (III) mixing the pelletsproduced in (I) and the pellets produced in (II) in amounts sufficientto provide a vulcanizable composition when massed.